This invention relates in general to heat processable media imaging systems and more particularly to such systems that have reduced maintenance costs and more reliable image quality performance.
One way of heat processing photothermographic media in a processor is to bring it in contact with a heated drum. To maintain good media contract with the drum a plurality of hold-down-rollers can be used to hold the media against the drum. During processing, some of the silver behenate in the media emulsion is converted to silver behenic acid. Some of the behenic acids and other byproducts of the processing reactions can leach through the media emulsion overcoat. These escaped chemicals are referred to as FAZ. At processing temperatures, the escaping FAZ is liquid. The FAZ ends up coating the drum surface where the media makes contact. After the media leaves the drum, the FAZ liquid left behind goes through changes. If the media hold-down-rollers contact the drum surface after the media has passed, some FAZ transfers to the contacting roller surfaces. Retained coating solvents and other light molecules in the FAZ quickly evaporated off the FAZ liquid surface. The remaining less volatile molecules can continue to slowly react with one another, with gasses diffusing in from the liquid/air surface and with the solid surfaces on which the FAZ clings. In some cases, FAZ reactions can form solid particles which can either remain in suspension in the FAZ liquid or deposit on the drum or rollers. FAZ, which is not attached to the surfaces as a solid deposit, is subsequently diluted by FAZ leaching from the next sheet of media processed. Some of this FAZ mixture leaves the drum as a surface deposit on the media overcoat. Ideally, the system would reach a steady state condition in which the quantity FAZ leaching out of the media equals the evaporation quantity plus the quantity exiting the processor on the surface of the sheet of media. This is often not the case and eventually some solid deposits form on the drum and/or roller surfaces.
In processors where media hold-down-rollers contact the FAZ coated part of the processing drum, the solid surface area with which FAZ interacts approximately triples. Consequently, the FAZ surface exposed to air also. Roller/drum contact also introduces mechanical working of the FAZ surface deposits. The contacting roller nips also serve to repeatedly force dust and other dirt particles brought into the processor on the media to be repeatedly pressed against the drum and roller surfaces, increasing their chance of sticking to one or the other and becoming nucleation sites for FAZ deposits. Small but frequent thermal cycling of FAZ is introduced as the FAZ on the heated drum and cooling rollers pass through their contact nips where heat is exchanged. FAZ on the rollers also sees much larger cooling cycles as media comes through, cooling the rollers significantly. All of these factors can serve to drive chemical reactions which might be absent in an undisturbed FAZ drum coating.
Although stepped drums or rollers have been used in reproduction systems (see: U.S. Pat. No. 5,465,146, issued Nov. 7, 1995, inventors Higashi et al.xe2x80x94Stepped Roller in a Fuser; U.S. Pat. No. 5,583,556, issued Dec. 10, 1996, inventor Kimxe2x80x94Stepped Drum in a Thermal Transfer Type Printer; U.S. Pat. No. 6,042,228, issued Mar. 28, 2000, inventors Yamada et al.xe2x80x94Stepped Rollers for Feeding Roll Sheets in an Image Forming Apparatus), none are suitable for solving any of these problems.
According to the present invention, there is provided a solution to these problems. The frequency and time required for preventive maintenance of a photothermographic processor can be significantly reduced by the use of stepped rollers or of a stepped drum design. In such a system the rollers and drum surface are gapped and do not touch each other in the central zone where they make media contact during media transport. This prevents the media hold-down-rollers from touching the processing residue coating (FAZ) on the drum. In either of these designs, an undercut that is about ⅝ the thickness of the media to be transported is used either on the drum or on the rollers in the media contact area. During standby, while film is not being processed the drum and rollers are gapped and only make rolling contact on either side of the film path. As film is transported through such a processor, it lifts the rollers by about xe2x85x9c the media thickness, providing the full clamping force of the rollers against the film to ensure contact of the media against the hot processing drum.
According to a feature of the present invention, there is provided an apparatus for processing heat processable media comprising: a rotatable heated drum for processing heat processable media, having width and depth dimensions; and a plurality of rollers spaced around a portion of the periphery of said drum and in contact therewith, said rollers holding said media to said drum; wherein at least one of said drum or at least some of said plurality of rollers have a channel in a central region thereof, said channel having a width greater than said width dimension of said media and a depth less than said thickness dimension of said media.
The invention has the following advantages.
1. Reduced maintenance time and costs.
2. More reliable image quality performance.
3. Increased life of system components.
4. Reduced need for strong chemical cleaners